The invention relates to a driving circuit of a flat panel display and, in particular, to a driving circuit with a pre-charge function.
A conventional active pixel driving circuit comprises a gate line, a data line and a pixel array. Each pixel is controlled by a thin film transistor. A low temperature polysilicon process is employed to integrate vertical and horizontal scan shift registers on a glass substrate. A vertical scan shift register comprises a shift register and a gate line. A horizontal scan shift register comprises a shift register, a switch and a data line. A location of a pixel to be charged is determined by a combination of signals from both vertical and horizontal scan shift registers. When resolution of a display is increased, charge time of a pixel reduced and the pixel is not completely charged.
Some patents already provide solutions to the mentioned problems. FIGS. 1A to 1C and FIGS. 2A and 2B, respectively, show embodiments of U.S. Pat. Nos. 5,892,493 and 6,731,266. In those embodiments, an additional pre-charge circuit (marked by the square) is utilized to pre-charge a pixel before an actual signal reaches the pixel. When the actual signal reaches the pixel, it takes time to fully charge the pixel due to a small difference between the pre-charge voltage and the actual signal. However, the additional pre-charge circuit generally requires an additional pre-charge control signal and pre-charge voltage. As a result, more thin film transistors are required and the architecture is more complicated.
FIG. 3A shows a structure of a conventional horizontal scan shift register. The horizontal scan shift register comprises shift registers SR1 to SR3, a bidirectional circuit Bi-direc and a transmission gate TG. A start pulse signal HST is transmitted to a bidirectional circuit Bi-direc and the bidirectional circuit Bi-direc selects a direction to scan and provide input pulses to the shift registers SR1 to SR3. The shift registers SR1 to SR3 generate output pulses via shifting input pulses by a clock width. The output pulses HSR1 to HSR3 sequentially turn on switches such that data signals are stored into the pixels via RGB signal lines.
Since the conventional shift register cannot pre-charge, performance of a display cannot be improved. If a pre-charge function is needed, an additional circuit block for pre-charge is required, as shown in FIG. 3B.